Prediction of the Reverberation Time in Rectangular Rooms with Non-

Uniformly Distributed Sound Absorption

R. Neubauer1, B. Kostek2

1Consulting Bureau, Ingolstadt, Germany

2Sound & Vision Engineering Department,

Faculty of Electronics, Telecommunications and Informatics,

Technical University of Gdansk, PL

SUMMARY

The aim of this paper is first to review the best known reverberation time formulae and then to

show that the reverberation time cannot be thereby predicted accurately in cases mostly

encountered in practice, where the sound field is not diffuse. Introducing a correction to

Fitzroy’s formula allows better prediction of the reverberation time in the case of non-

uniformly distributed sound absorption. Comparison of calculation results obtained on both

the basis of classical equations and the new time reverberation formula introduced is shown.

In addition, the results obtained by measuring reverberation conditions in situ and those

predicted for the same enclosure are compared and conclusions drawn.

1. Introduction

There exist many parameters that describe acoustic quality of an enclosure. The importance of

some has been already established in the pioneering study of Beranek [4] and later by other

researchers [3][32][33][38], but there is still no consensus on a set of parameters that should

be taken into account while describing the acoustical quality of a room [22]. This is due to

differences in functionality of a given room, volume of rooms, distribution of absorption, etc.

[2][27][32][33]. A so-called optimum reverberation time can serve as an example of such

problems. Optimum values of this acoustical quantity that can be found in many literature

sources differ to a large extent, which was pointed out by Straszewicz in his paper [40]. In his

opinion a pronounced divergence of opinions makes doubtful whether it is possible to assign

the optimum reverberation time related to the given volume of a room or to the kind of sound

produced only [40]. Numerous research studies show that rather than trying to achieve the

optimum reverberation time for a given room, it is better to govern other acoustical

parameters that influence acoustical quality. This is especially true in the case of the

multifunction interiors. Niemas, Sadowski and Engel formulated some requirements as to

designing and estimating acoustical properties of sacral enclosures [27]. They pointed out the

importance of other quantities than the reverberation time in the evaluation of the acoustical

quality of such interiors [27]. Problems related to designing and estimating acoustical

properties of interiors were also reviewed by other researchers [39][42][43], among others by

Rakowski [30] and Sadowski [32][33]. In addition the relationship between acoustical

parameters measured in a room and the acoustic quality assessed subjectively is still

researched [17][22]. However as was pointed out by Vorländer, one of the most relevant

sensations of the sound field in rooms is still the cognition of reverberation [42].

Reverberation appears to be responsible for the impression of being in a room as well as

providing an awareness of distance from the source, whereas for example spatial impression

due to lateral reflections appears to be more a source-specific effect involving the feeling to

be close to the listener [3]. As Budzynski [8] pointed out, early reflections from sidewalls are

also responsible for the effect of increasing auditory distance localization.

During the past century several formulae for predicting reverberation time were

developed empirically and theoretically, based on the assumption of homogeneous repartition

of sound energy within the room, and consequently uniformly distributed sound absorption.

The problem of the reverberation time prediction for non-uniform distribution, however,

remains so far, open for discussion and for finding solutions fitted better to practical

application.

The first and most remarkable approach to describe the reverberation characteristics of

an enclosure was found by W.C. Sabine [31] around 1900. Sabine established his theory on

the basis of practical results. To find a theoretical basis for calculating reverberation time,

many researchers contributed new theories. Since Sabine published his results, several

different approaches have been adopted to obtain equations that describe the reverberation

characteristics. Among others, the best known researchers, who developed theories of

reverberation include: Franklin (1903) [14], Jaeger (1911) [15], Fokker (1924) [13],

Buckingham (1925) [7], Schuster and Waetzmann (1929) [36]. Then, Eyring (1930) [11]

presented his remarkable paper, followed shortly after by Millington (1932) [24] and Sette

(1933) [37]. After a further 25 years Fitzroy (1959) [12] published an empirical solution for

reverberation time prediction in non-uniform rooms, his paper, however, went almost

unrecognized at that time and was rather negatively perceived. In the last 30 years, Schroeder

(1965) [34], Kosten (1965) [19], Cremer and Müller (1978) [9] Kuttruff (1975) [20], Nilsson

(1992) [28], Tohyama et al. (1995) [41] added some new issues to the theory of reverberation.

In 1988 Arau [1] presented an improved reverberation formula taking into account the non-

uniform distribution of sound absorption. Lately, papers by Kutruff [21] and Bistafa and

Bradley [6], which dealt with the similar problems, appeared. Recently, within the framework

of the standardization process an improvement of the estimation of the reverberation time in

rooms with irregular absorption distribution based on Nilsson’s model was proposed and

discussed [23][28].

A general description of the reverberation time based on Sabine’s reverberation theory is

still in common use. However, in the case of a room in which sound absorption is not

uniformly distributed, the reverberation time frequency characteristics cannot be predicted

accurately using Sabine’s or other classical reverberation theories. These theories are based on

the assumption that the sound field considered is completely diffuse. This will, in general, be

sufficiently diffuse if there are no large differences in the basic dimensions of the room, walls

are not parallel, sound absorbing material is uniformly distributed, and most interior surfaces

are divided into parts. In practice, almost none of these requirements is fulfilled. In 1959,

Fitzroy introduced an empirically derived equation that considers non-uniform distribution of

absorption. However, a thorough investigation of Fitzroy’s equation revealed that in most

cases the predicted reverberation time was generally too long. It is worth noting that

acousticians are not altogether satisfied with existing formulae on reverberation time, thus the

European standard concerning this issue is still an open question [10]. Furthermore, the

reverberation time is an important parameter in calculating other objective descriptions of

room acoustics. Since such parameters are well related to subjective assessment descriptors of

room acoustics, it is most important for the acoustical design process to provide a general

design tool which enables prediction of relevant sensations in the stage of planning a certain

type of room [5][17].

In the following paragraphs a review on best known formulae for predicting

reverberation time is presented, along with corresponding calculations results. This is shown

for the case of rectangular enclosures, because in such cases it is possible to systematically

study the reverberation time related to the room volume.

2. Time Reverberation Formulae

Most of parameters that describe characteristics of an acoustic hall are calculated basing

on the assumption that it is usually sufficient to consider the propagation of sound energy and

not sound pressure or particle velocity, therefore all phase effects can be neglected. The basis

of this assumption is that the dimensions of a hall should be large enough in comparison with

the acoustic wavelengths. The so-called Schroeder cut-off frequency [35]:

[Hz]/2000 VT=fS (1)

limits the frequency under which this assumption is not justified. That is, fs can be considered

as the lower limit of frequencies at which a statistical treatment of superimposed normal

modes in a room is permissible. In contrast, below the Schroeder cut-off frequency, the

resonance peaks of the sound field are insufficiently dense to be analyzed statistically.

One of the most important parameters describing the quality of the room is the

reverberation time. According to the classical formula reverberation time is defined as time

needed to decrease energy by 60 dB from its original level after instantaneous termination of

the excitation signal. This parameter, originally introduced by Sabine, is given by equation

(2):

16060 V/A.=T ⋅ (2)

where: V - hall volume [m3], A - total area of absorption [m2], 0.16 - coefficient introduced

first empirically, depending on propagation conditions. Therefore in the literature, values

equal to 0.16, 0.161, 0.162, 0.163, 0.164 can be found. It is also worth noting that Sabine

derived experimentally a value of 0.164. In this paper, contrarily to the original values that

can be different, a value equal to 0.16 will be used in calculations, since such an accuracy is

sufficient for comparison purpose.

If the surface area of the room is S, Sabine’s average absorptivity α is defined by:

α = A/S (3)

and Eq. (2) assumes the form:

α⋅⋅

=S

VT 16.060 (4)

In order to complete Sabine’s formula by taking into account the attenuation constant m of air,

which takes into account the attenuation of sound during its free propagation, one obtains:

mVS

VT4

16..060 +⋅

⋅=

α (5)

As is seen from Equation (4), T60 may be controlled by either a change in volume, or the

absorption factor. Equation (4) assumes that the sound energy is equally diffused throughout

the room (i.e. homogeneous and isotropic). Actually, this condition is rarely fulfilled due to

the large areas existing in a hall characterized by differentiated absorption. Therefore, in

practice, there are several formulae describing the reverberation time.

It was discovered by Eyring that the classical formula given by Sabine is not fulfilled

when there is considerable room absorption. Eyring pointed out in his paper that Sabine’s

formula is essentially a “live” room formula and that the reverberation time is shape-

dependent [4]. He presented the revised theory thoroughly and derived a form of the

reverberation time equation, which is more general than Sabine’s formula.

Eyring’s formula is based on the mean free path between reflections [29][44]. The mean

free path in an enclosure characterized by a diffuse sound field is as follows [16][18]:

SVl ⋅

=4 (6)

where: V is the total volume of a room [m3], and S - as before.

The use of the above Eq. (6) yields:

)1ln(16.0

60 α−⋅−⋅

=S

VT (7)

with:

ii

iSS

αα ⋅⋅= ∑1 (8)

Eyring assumes that sound coming from a source in a room is successively reflected by

boundaries having an average coefficient α . Each time a wave strikes one of the boundaries,

a fraction (α ) of the energy is absorbed, and a fraction (1-α ) is reflected. The number of

reflections per second is numerically equal to the distance sound will travel in one second

divided by the average distance between reflections. As is seen from Eq. 7, Eyring assumed

that energy attenuation in this case is nEyring )1( α− per second, where boundE SA /=α and

Sbound are bounding surfaces. Eq. (7) is known as Eyring’s reverberation formula.

Additionally, Millington [24] and Sette [37] derived an equation to predict the

reverberation time that is based on similar assumptions as Eyring’s formula. It differs,

however, in the way in which absorption coefficients of the various portions of a wall are

averaged. This leads to the Millington-Sette’s formula:

∑ −⋅−⋅

=

iiSi

VT )1ln(

16.060 α

(9)

which, in the limit of all αi << 1, reduces to Sabine’s formula with αi = α(Eyring)i.

On the other hand, Fitzroy states in his paper [12] that it is possible to take into

consideration not only physical, but also geometrical aspects of a sound field in an enclosure.

In this way the sound field may tend to settle into a pattern of simultaneous oscillation along a

rectangular room with three major axes - vertical, transverse, and longitudinal. The solution

appeared to lie in some relationship within the three possible basic decay rates along these

axes, each being influenced by the different average absorptivities normal to these axes, each

rate being unique within its specific axis. In a rectangular room, one may consider three sets

of parallel boundaries. If energy oscillates simultaneously between each pair of boundaries the

average absorption in each pair would control sound waves travelling between that specific

pair during the sound decay period.

It should be remembered that the Eyring's formula assumes the absorptivity to be equal

in all directions and Fitzroy's equation in the case of a rectangular room, takes into account

three- dimensional geometry. Thus Fitzroy derived empirically an equation in which non-

uniform distribution of absorption is assumed.

−−

+−−

+−−

⋅=)1ln()1ln()1ln(

16.0 260zyx

zyxSV

Tααα

(10)

where: x, y, z - total areas of two opposite parallel walls in m²,

αx, αy, αz - average absorption coefficients of a pair of opposite walls,

S - total surface area of the room in [m²],

V - total volume of the room [m3].

Tohyama and Suzuki presented the “almost-two-dimensional” diffuse field theory in his

paper [41]. The physical space inside boundaries is three-dimensional. In acoustics, there are

cases, where two-dimensional acoustic field is better suited to estimate reverberation

conditions. It may occur in a concert hall with hard walls perpendicular to an absorbent floor.

Reverberation time in a two-dimensional diffuse field determined by “the later part of the

energy decay”, assuming sound velocity equals 340m/s, becomes [41]:

)(lnLS.

Txyα−⋅−

⋅=

11280

60 (11)

where L is the total length of the two-dimensional space and xyα denotes absorption

coefficient in the xy-two-dimensional field.

A small regular-shaped room with a carpet-covered floor can be treated as the sound

field between two- and three-dimensional [41]. In the case of a two-dimensional field

reflections at the z-walls are neglected. For small-sized rooms, however, characterized by

almost two-dimensional diffuse field z-walls reflections are taken into account. This field is

assumed to be composed of tangential, oblique and "almost-tangential" waves. By replacing

xyα with the averaged absorption coefficient xyAI−α characterizing the almost two-

dimensional diffuse field, Eq. (11) becomes:

)1(ln128.0

xyALxyxyAL L

ST−

− −⋅−⋅

=α

(12)

where:

µαµαα ⋅+−=− zxyxyAL )1( denotes the averaged absorption coefficient,

zα - averaged absorption of the z wall,

xyα - averaged absorption in the xy-two-dimensional field,

Lxy=2(LxLy) – circumference in [m], Lz – length in [m],

)fL/(cm z 02 4 ⋅⋅⋅=µ , xyxy LSm /⋅= π ,

m - mean free path,

c - speed of sound in air,

f0 - wave frequency.

In his papers, Arau introduced a model of calculating the reverberation time for the case

of asymmetrical distribution of absorption, assuming that the reverberation decay is a

hyperbolic process [1]. This decay is a superposition of three contributions: initial decay, first

and second linear portion of the decay, and the third linear portion. Arau considered the

reverberation time of a room to be equal to the area-weighted geometrical mean of the

reverberation periods in each of the rectangular directions. The absorption coefficients used in

his formula are the average absorptivities of each pair of opposite walls.

( ) ( ) ( )

+−⋅−⋅

+−⋅−⋅

+−⋅−=

mVS

V SS

mVS

V SS

mVS

V SS

Tzyx

zyx

41ln

16.0

41ln

16.0

41ln

16.0

ααα (13)

where: αx is the area-weighted arithmetical mean of the energetic absorption coefficients of

the

floor Sx1 and ceiling Sx2 surfaces;

and Sx = Sx1 + Sx2

αx = (αx1Sx1 + αx2Sx2)/Sx

αy and αz are the area-weighted arithmetical mean of energetic absorption

coefficients of the side-walls and front- and end-walls, respectively.

S = Sx + Sy + Sz is the total area of the room surfaces,

m is the molecular absorption coefficient of air.

If the enclosed space has a non-regular distribution of absorption, irregular shapes or is

filled in, to a large extent, with e.g. equipment, decorative elements, etc., the predictions of

the reverberation time should be based on the Nilsson model [28]. This may improve

predictions of the reverberation time for the irregular absorption distribution. An essentially

rectangular space with irregular absorption distribution is quite common. In many office

rooms absorption is applied only to the ceiling, all other surfaces being reflective. In such a

case classical time reverberation formulae rarely solve the problem. Nilsson proposed that the

sound field should be divided into the most characteristic part, i.e. tangential to the considered

surface, and remaining parts of room surfaces. A practical approach based on that model, but

making use of absorption data measured according to standard methods, is presented in the

Annex D of the European Standard prEN 12354-6 [10].

Kuttruff considers the case of the partially diffuse field within the room and introduces

the concept of the reflection coefficient ρ = 1 - α. Basing on the assumption that absorption

coefficient α and hence ρ are independent of the angles, he made use of Lambert’s law of

diffuse reflection. By focussing on the overall reverberation time, and neglecting details of the

decay process and additionally under the assumption of an exponential law for the time

dependence of the irradiation strength over the whole surface of reflecting walls, he defined

an absorption exponent α*, which is valid under the aforementioned assumption [20]. The

assumption of an exponential law is reasonable since, at least in rectangular rooms, the decay

process of the sound energy will decrease exponentially.

Kuttruff introduced a correction to Eyring’s formula and showed that the absorption

exponent α* would assume its Eyring’s value [20]:

αEyring = - ln ρ = -ln(1-α ) (14)

if the irradiation strength was constant. This is true if ρ and hence the absorption coefficient α

has everywhere the same value. In general, the effective absorption exponent will be smaller

or larger than (-ln ρ ), depending on the room shape and the distribution of the wall

absorption.

−−

∑

−−

++

−=

Σ

SS

S

nn

n nnn

222

2

* 1ln1ln

ρρ

ρρρ

ρα (15)

where: α−=ρ n1 denotes the average reflection coefficient of surface area Sn,

S is the total surface area of the room in [m²].

In most cases the second term in the denominator is much smaller than the first and hence can

be neglected. Expanding the second logarithm into a power series and neglecting all terms of

higher than first order gives:

−

∑

−−

+≈

S

Sn nnn

Eyring

ρ

ρρραα

2

2

* (16)

Inserting Kuttruff’s correction to Eyring’s reverberation formula and completing this

formula by taking into account the attenuation constant m of air, leads to:

[ ] mVSV

T 4)1(ln16.0

60 +∆+−−⋅⋅

=α

(17)

where ∆ is:

( )

( ) ∑

∑⋅−

⋅−=∆

iii

ii

ii

SS

S

222

2

ρρ

ρρρ (18)

Kuttruff showed that his correction to Eyring’s formula could easily be applied where n-

1 surfaces have nearly the same reflection coefficient and one surface, namely the nth surface,

e.g. the floor where the audience’s seats are placed, is characterized by a different absorption

coefficient. In this case very good conformity with the computer simulated results may be

obtained. On the other hand, if a room has asymmetric absorption Eyring’s modified formula

is considerably incorrect. In this case in order to overcome this problem one can follow the

reasoning presented in Pierce’s book [29].

3. Modification of Fitzroy’s Equation

In the study conducted, correction to Fitzroy’s equation has been introduced in order to

approximate calculated time reverberation values closer to the measured ones. A suggested

modification of Fitzroy’s equation was first presented in a paper by Neubauer [25]. Since

Kuttruff [20] introduced a correction to Eyring’s formula and Fitzroy’s equation [12] is based

on Eyring’s concept, it seems possible to introduce a similar correction to Fitzroy’s equation.

This can be done to achieve more accurate approximation of the reverberation time in cases

where the main absorption is of the floor and the ceiling. This modification leads to Fitzroy’s

modified equation, called here a New Formula, which is presented and discussed in this

paragraph. It has been proven that the reverberation time according to Fitzroy’s modified

equation is useful in cases where the sound absorption on opposite sides is substantially

higher than on the remaining room surfaces. As this is the case of a large number of existing

rooms, for practical use one may modify Fitzroy’s equation by splitting Kuttruff’s correction

into two parts, namely the part of the ceiling-floor and the part of the remaining walls. This

reasoning will be presented later on.

Because examples that Fitzroy presented in his paper conform with the three-term

formula, it is worthwhile attempting the same manner of reasoning. Compared calculation

results however, reveal no advantage over the more simple equation provided by Sabine or

Eyring. In particular, simulations for a room different from those given as examples in

Fitzroy’s papers, generally result in higher values than these based on Sabine’s or Eyring’s

formulae. For the most important practical case where either the ceiling and/or the floor are/is

highly absorptive it results in the “almost two-dimensional” field. Generally, this occurs in a

real room when the absorbing capacity of the ceiling and floor exceeds that of the remaining

walls. For such a case Fitzroy’s equation may be rewritten using an appropriate modification.

By dividing the room surfaces into the floor and ceiling areas, and the remaining wall

areas one obtains the following expressions:

Scf = 2·l·w (19 a)

Sww = 2·l·h + 2·h·w (19 b)

Stotal = 2·[h·(l + w) + l·w)] (19 c)

where are h, l, w - as defined before.

Introducing Eq. 19a to 19c one may rewrite Fitzroy’s equation and furthermore by using

Kuttruff’s correction from Eq. 18 it yields a New Formula:

⋅++⋅

⋅

= **260 )(32.0

cfww

wlwlhS

VT

αα (20)

where: V, S - volume in m³ and total surface area of the room in [m²],

h, w, l - room dimensions: height, width and length in [m],

*wwα ,

*cfα - average effective absorption exponent of walls, ceiling and floor,

respectively:

( ) ( ) ( )( )[ ]

2 2 2w1 w2 w3w1 w1 ww w2 w2 ww w3 w3 ww

2www1 w2 w3 w4ww

...1 S S S* lnS S S S

ρ ρ ρ ρ ρ ρ ρ ρ ρα

ρ ρ

− + − + − + = + + + +

(21a)

( ) ( )[ ]

+

⋅−+⋅−+

−= 2

22

*

)(1ln

cfcf

fcfffccfcccf

SS

SS

ρ

ρρρρρρρ

α (21b)

where: α - arithmetical mean of the surface averaged absorption coefficient,

( )α−= 1ρ - reflection coefficient.

A comparison of calculated results using the new Sabine’s and Eyring’s formulae is shown in

Fig. 1. From Fig. 1 good conformity between all three formulae can be noted, although both

Eyring reveals about 1% smaller values of the calculated reverberation time compared with

the New Formula. In the consecutive figure (Fig. 2) results are presented for the case where

the floor is characterized by high absorption and the remaining surfaces have low absorption.

The absorbing materials have an absorption coefficient for the floor of 0.80 that for ceiling of

0.2 and other surfaces of 0.02.

In this case it can be observed that the differences in the predicted reverberation time

are not negligible. Both Sabine’s and Eyring’s formulae yield from about 25% to 35% higher

values of the calculated reverberation time compared with the New Formula. This is due to

unevenly distributed absorption in the room. Since both Sabine and Eyring assumed that the

sound field is diffuse and the sound absorption regularly distributed throughout the room,

such a result is not surprising. Therefore, the advantage of using the New Formula is

especially important in cases of irregularly distributed absorption. In the next figure (Fig. 3)

results obtained for a room with regularly distributed absorption, but also a highly absorptive

one is shown. In this case all surfaces have a sound absorption coefficient of about 0.40. From

Fig. 3 it can be clearly observed that in the case of high, but evenly distributed absorption,

results obtained are nearly the same by using both Eyring’s and the New Formula. However,

comparing the results more thoroughly reveals a deviation of about 0.16%. Values of the

estimated reverberation time are smaller using Eyring’s formula as compared with the new

one. It may therefore be assumed that the New Formula is better applicable than Eyring’s

formula, especially in cases of unevenly distributed absorption in the room.

4. Comparison of Reverberation Time Predictions Resulting of a New Formula and

Other Existing Formulae

In this paragraph the best known formulae for predicting reverberation time are

compared with the New Formula. Investigation of the prediction model for estimating the

reverberation time in rectangular rooms is focused on comparison of the following formulae

of: Eyring, Millington-Sette, Tohyama, Arau, model contained in the European Norm (Annex

D, prEN 12354-6) and Fitzroy. Although Tohyama’s formula should be properly referenced

as also to Suzuki [41], but for the purpose of result presentation this is abbreviated to

Tohyama's formula only.

Since, in many practical cases, the assumption of diffuse field conditions by applying

Sabine’s theory does not conform with the true room absorption distribution, it is of practical

interest to develop a proper formula to estimate the reverberation time even for non-uniformly

distributed sound absorption in the room.

In order to show the effect of computing reverberation time using various formulae,

some calculation results are presented in the following figures (Fig. 4-6). First, two cases of

the absorption coefficient value at the limits, i.e. a “live” room with little absorption and a

“dead” room with high absorption will be discussed. In the following calculations the term

4mV, which represents air absorption is omitted.

The simulations presented in this paragraph are performed for the case of a rectangular

room of dimensions l, w, h (consecutively: length, width, and height). The rectangular shape

of the enclosure room was chosen because of its systematically treatable dimensions.

However, there is no need to reduce the findings of the carried out investigation to rectangular

rooms only, as long no elliptical or circular rooms are considered. Moreover, the European

Norm prEN 12354-6 does not distinguish between rectangular and irregular shaped rooms.

The investigated room volumes were from 50m3 to 8000m3. The volume range used in

experiments was chosen arbitrarily. But one should bear in mind that for very large rooms,

especially if the ceiling is low, the RT theory is not longer suitable.

In the first figure (Fig. 4) the difference between various calculation formulae when all

surfaces have very small absorption (live-room condition) can be seen. It is obvious because

the diffusion in such a case is good. It can be observed that there is no meaningful difference

in the computed reverberation time values based on various formulae except that using both

Tohyama’s formula and the model contained in the European Norm (Annex D, prEN 12354-

6). In the next figure (Fig. 5) the difference between various formulae calculation results may

easily be shown where all surfaces are of a high absorption (dead-room condition). It is also

seen that the model from the European Norm, Sabine’s and Tohyama’s formulae are

discrepant compared to other reverberation formulae.

It can be noted from Fig. 5 that all but the model from the European Norm, Sabine’s and

Tohyama’s formulae yield more or less the same computed reverberation time values.

Simulations based on the model contained in the European Norm yield the highest values

followed by those calculated on the basis of Sabine’s and then Tohyama’s formulae.

In some respect it may be interesting to compare results when only one surface is highly

absorptive and all others are characterized by low absorption. This may happen if the floor is

covered with seats, but walls and ceiling are bare. In this case the average absorption

coefficient resulting from audience seats is over 0.8 and the other surfaces of about 0.02 yield

results which are depicted in the consecutive plot (Fig. 6). It can be seen from Fig. 6, that

Fitzroy’s formula followed by Arau’s equation produces the highest values of the

reverberation time. Based on Tohyama’s and Sabine’s equations one may obtain close results

followed by Eyring’s equation based values. Millington-Sette's formula provides the lowest

values.

As already mentioned, results shown in Fig. 6 are derived by using an absorption

coefficient of the floor of 0.8 (αcf=0.43) and all other surfaces 0.02. This yields an overall

absorption coefficient dependent on the method of averaging, whether by means of arithmetic

or geometric averaging. Using the arithmetical mean yields an absorption coefficient of 0.24 –

0.22 depending on the room volume and on the other hand using the geometrical mean (for

Millington-Sette) this operation results in an absorption coefficient of 0.46 – 0.43 also

depending on the room volume.

5. Differences between Measured Results and Predicted Values

In order to observe the quality of the New Formula in the case of non-uniformly

distributed absorption, one should compare results of calculations based on various time

reverberation formulae with values of the reverberation time values measured in situ.

For practical reasons, reverberation time measurement was performed in unoccupied

rooms (no audience present). Since sound absorption coefficients of the surfaces are rarely

known in places already built, thus in order to compare measured with calculated values it is

necessary to estimate the individual sound absorption coefficients of respective surfaces of the

room. This can be done basing on two assumptions. First, 10% accuracy is often adopted in

engineering applications aimed at reverberation time predictions. Secondly, as shown by

Bistafa and Bradley [5] and other researchers any absorption coefficients enable prediction of

reverberation time of a bare room within this 10% range accuracy. Thus it may be assumed

that the adequate values of the unknown sound absorption of the room surfaces can be

obtained from measurements using either Sabine or Eyring classical formula. Such an

assumption can be also found in norms (e.g. DIN 52212, DIN EN 20354, ISO 354).

Moreover, Neubauer estimated the error that is caused by such an approximation procedure

and showed that it can be neglected [26]. In the experimental procedure Sabine formula was

adopted. This was done by “calibrating” the calculated reverberation time using Sabine’s

formula and comparing the obtained results with the measured reverberation time yielding the

respective sound absorption coefficients. The individual sound absorption coefficients were

then used to calculate the respective reverberation time. Such methodology is often adopted in

engineering applications and has proved satisfactory [6].

5.1. Measurement Procedure

In order to measure the reverberation time in situ, measurement procedure was

performed according to the requirements of the German Standard DIN 52216 (ISO 344) or

German Standard DIN EN 20354 (ISO 354). Rooms in which the measurements took place

were exposed to the “pink noise” signal filtered in the 1/3 octave band center frequency range

between 50 Hz and 5 kHz. Measurements were taken at several, at least 3 locations with at

least two microphone positions. The experimental configuration consisted of a ½”

microphone and noise source mounted inside the room. The subsequent analysis was achieved

conveniently on a dual channel Real-Time Analyzer: Norsonic RTA Type 840-2. The

reverberation time was calculated after the standard ISO/DIS 3382 at a rate of decay given by

the linear least-squares regression of the measured decay curve from a level 5 dB below the

initial level to 35 dB below. The reverberation time was calculated automatically by

evaluating ensemble-averaged decay curves using linear regression.

It should be remembered that the reverberation time is defined as time needed to

decrease energy by 60 dB from its original level after instantaneous termination of the

excitation signal, but since this dynamic range is often not obtainable in measurements, in

practice, reverberation time measurements are based on the upper 20 or 30 dB of the dynamic

range.

5.2. Prediction of the Sound Absorption Coefficient Values

The measured reverberation time (T60) at 500 Hz was introduced to Sabine’s formula in

order to find the absorption coefficient. The accuracy obtained is shown in the graph in Fig. 7.

Figure 7 shows good conformity between "approximated" and measured reverberation

time values. One of the causes of the observed difference is due to the fact that the absorption

coefficients were approximated using second decimal order. Such a way of reasoning was

also used by Bistafa and Bradley [6] and proved to be effective in cases of unknown

absorption coefficients of a room in which the reverberation time was measured. Based on

such an approximation procedure the room-related individual sound absorption coefficients

were taken then in order to compare calculated reverberation time values using various

formulae. The obtained sound absorption coefficient was used for all formulae, i.e. all

surfaces have the same respective "approximated" sound absorption coefficients.

In order to simplify calculations, it was assumed that the averaged sound absorption

coefficient of the walls was equal to 0.05. Since all room dimensions are measured, one can

use Sabine’s equation to evaluate the measured reverberation time to obtain the averaged

sound absorption coefficient of the ceiling and floor. Such calculations for a frequency band

of 500 Hz are given in Tab. I. All notations in Tab. I are as previously defined. It can be

observed that averaged absorption coefficient values of the floor and ceiling are in most cases

at least one order higher than those of the room walls.

Table I. Sound absorption coefficient values resulted from the approximation procedure (500

Hz frequency band)

Length

[m]

Width

[m]

Height

[m]

Volume

[m3]

Sww

[m2]

Scf

[m2]

S

[m2]

wwα cfα α αAL-xy RT [s]

(Measured)

4.45 3.30 3.55 52.13 55.03 29.4 84.40 0.07 0.10 0.08 0.08 1.25

5.19 3.72 3.215 62.07 57.29 38.6 95.90 0.04 0.11 0.07 0.07 1.44

5.80 4.60 2.55 68.03 53.04 53.0 106.40 0.04 0.06 0.05 0.05 1.91

6.30 5.40 2.77 94.24 64.82 68.0 132.86 0.20 0.09 0.14 0.12 0.80

7.12 6.89 3.04 149.13 85.18 98.1 183.29 0.06 0.10 0.08 0.09 1.28

9.24 7.00 3.00 194.04 97.44 130.5 227.80 0.10 0.32 0.23 0.30 0.62

16.00 7.52 3.15 379.01 148.18 236.9 388.82 0.09 0.35 0.25 0.39 0.67

15.00 10.00 5.00 750.00 250.00 300.0 550.00 0.05 0.28 0.17 0.23 1.19

18.65 17.00 3.00 951.15 213.90 632.4 848.00 0.07 0.24 0.19 0.45 1.30

17.20 16.00 6.00 1651.20 398.40 550.4 948.80 0.02 0.15 0.10 0.15 2.25

5. 3. Comparison of Measured and Predicted Results

Using the obtained sound absorption coefficient one can compare the results from

measurements with those of calculations (predicted results). Such calculations were

performed for the whole frequency band characteristics, however for comparison purpose all

values presented in Figs. 8 and 9 are shown only for a mid-frequency range of 500 Hz. The

obtained results are given for two ranges of room volumes. Although values given by

Eyring’s formula are close to the measured ones, those obtained on the basis of the New

Formula conformed better with the “real values” of the reverberation time. In addition, it may

be observed from Figs. 8 and 9 that by using Fitzroy’s, the model contained in the European

Norm (Annex D, prEN 12354-6), Arau’s and Tohyama’s equations no proper approximation

to the measured reverberation time values is obtained.

To summarize the obtained results it can be observed that the reverberation time values

calculated using the model from the European Norm (Annex D, prEN 12354-6), Tohyama or

Fitzroy formulae significantly deviate from the measured values, whereas the New Formula

provides values within a range of approximately ± 28 % and is always within the same range

as the measured reverberation time.

6. Conclusions

Based on the study conducted, the following conclusions may be derived:

In the case of low absorption there is no meaningful difference in the computed

reverberation times except for those calculated using Tohyama’s formula and the model

contained in the European Norm (Annex D, prEN 12354-6). There exists a difference

between various calculation formulae results in the case of high absorption. It was also

observed that values calculated on the basis of the model from the European Norm,

Sabine’s and Tohyama’s formulae differ considerably as compared with the other

reverberation formulae.

Prediction based on the proposed New Formula generally conforms much better than

classical formulae with the reverberation time values obtained from in situ measurements.

This is also true for various room volume values.

In addition, predicted reverberation time using the New Formula reveals a shorter

reverberation time than measured. This is especially true in the case of higher octave band

frequencies. However, in the case of investigated data best results were observed below or

equal to the octave center frequency of 1 kHz, which is very important for obtaining

adequate reverberation characteristics.

7. References

[1] H. ARAU-PUCHADES, An improved Reverberation Formula, Acoustica. 65, 163-180

(1988).

[2] Archives of Acoustics, Proc. of the Conference on Measurements of Reverberation

Phenomena in Halls, (Warsaw, April 1967), 3, 2, 99-245 (1968).

[3] M. BARRON, A.H. MARSHALL, Spatial Impression due to Early Lateral Reflections in

Concert Halls: The Derivation of a Physical Measure, J. Sound and Vibration, 77, 2, 211-232

(1981).

[4] L.L. BERANEK, Concert and Opera Halls. How they Sound, Acoust. Soc. Amer., (1996).

[5] L.L. BERANEK, Concert Hall Acoustics - 1992, J. Acoust. Soc. Amer., 92, 1-39 (1992).

[6] S.R. BISTAFA, J.S. BRADLEY, Predicting reverberation times in a simulated

classroom, J. Acoust. Soc. Am., 108, 4, 1721-1731 (2000).

[7] E. BUCKINGHAM, Theory and Interpretation of Experiments on the Transmission of

Sound Through Portion Walls, Bur. Standards, Sci. Paper No. 506 (1925).

[8] G., BUDZYNSKI, Theory of the Reflective Localization of Sound Sources, Archives of

Acoustics, 11, 1, 13-24 (1986).

[9] L. CREMER, H.A. MÜLLER, Die wissenschaftlichen Grundlagen der Raumakustik,

Band 1 S. Hirzel Verlag, Stuttgart (1978).

[10] European Standard prEN 12354-6, (CEN/TC126/WG2 N241), final draft, May 2001.

[11] C.F. EYRING, Reverberation Time in “Dead” Rooms, J. Acoust. Soc. Amer., 1, 217-

241 (1930).

[12] D. FITZROY, Reverberation formulae which seems to be more accurate with non-

uniform distribution of absorption, J. Acoust. Soc. Amer., 31, 893-897 (1959).

[13] A.D. FOKKER, Over Den Nagalm, Physica, 4, 262-273 (1924).

[14] W.S. FRANKLIN, Derivation of Equation of Decaying Sound in a Room, Phys. Rev. 16,

372-374 (1903).

[15] A. JAEGER, Zur Theorie des Nachhalls, Wiener Akad, Ber., Math.-Naturwiss, Klasse,

Bd. 120, Abt. IIa, 613-634 (1911).

[16] W.B. JOYCE, Sabine’s reverberation time and ergodic auditoriums, J. Acoust. Soc.

Amer., 58, 3, 643-655 (1975).

[17] B. KOSTEK, Soft Computing in Acoustics, Applications of Neural Networks, Fuzzy

Logic and Rough Sets to Musical Acoustics, Studies in Fuzziness and Soft Computing, Physica

Verlag, Heidelberg, New York (1999).

[18] C.W. KOSTEN, The Mean Free Path in Room Acoustics, Acustica, 10, 245-250 (1960).

[19] C.W. KOSTEN, New Method for the Calculation of the Reverberation Time of Halls for

Public Assembly, Acustica, 16, 6, 325-330 (1965/66).

[20] H. KUTTRUFF, Nachhall und effektive Absorption in Räumen mit diffuser

Wandreflexion, Acustica, 35, 3, 141-153 (1976).

[21] H. KUTTRUFF, Energetic Sound Propagation in Rooms, Acustica – Acta Acustica, 83,

622-628 (1997).

[22] H. KUTTRUFF, On Acoustics of Auditoria, Institute of Technical Acoustics, Technische

Hochschule, 27-48, Aachen.

[23] Memorandum to CEN/TC 126/WG2, Possibilities to improve the estimation of

reverberation time in rooms, TNO Institute of Applied Physics, Netherlands, HAG-MEMO-

990076, 1-8 (1999).

[24] G. MILLINGTON, A Modified Formula for Reverberation, J. Acoust. Soc. Amer., 4,

69-82 (1932).

[25] R.O. NEUBAUER, Prediction of Reverberation Time in Rectangular Rooms with a

Modified Fitzroy Equation, ISSEM’99, Proc. 8th International Symposium on Sound

Engineering and Mastering, (A. CZYZEWSKI, B. KOSTEK, eds.), 115 – 122, Gdansk,

Poland (1999).

[26] R.O. NEUBAUER, Classroom acoustics - Do existing reverberation time formulae

provide reliable values?, 17th International. Congress Acoust., Rome, Italy (2001).

[27] M. NIEMAS, Z. ENGEL, J. SADOWSKI, Acoustic Issues of Sacral Structures, Archives

of Acoustics, 23, 1, 87-104 (1998).

[28] E. NILSSON, Decay process in rooms with non-diffuse sound fields, Report TVBA-

1004, Lund Institute of Technology, Depart. of Eng. Acoustics (1992).

[29] A.D. PIERCE, Acoustics, Acoust. Soc. of Amer., second printing, Chap. 6, 262 (1991).

[30] A. RAKOWSKI, Factors governing the acoustical quality of concert halls [in Polish],

Warsaw Academy of Music, 63-92.

[31] W.C. SABINE, Collected Papers on Acoustics, University Press Harvard, (1922);

Reprinted by Dover, New York (1964).

[32] J. SADOWSKI, Akustyka architektoniczna [in Polish], Arkady, Warszawa (1971).

[33] J. SADOWSKI, Akustyka architektoniczna [in Polish], PWN, Warszawa (1976).

[34] M.R. SCHROEDER, Some New Results in Reverberation Theory and Measurement

Methods, 5e Congres International D’Acoustique LIÉGE 7-14 Sept., G31 (1965).

[35] M.R. SCHROEDER, The Statistical Parameter of Frequency Curves of Large Rooms,

Acustica, 4, 594 - 600 (1954).

[36] K. SCHUSTER, E. WAETZMANN, Über den Nachhall in geschlossenen Räumen, Ann.

d. Physik 1 (1929), 671; also Textbook by Muller-Pouillet, on Acoustics Chapter VII, 456-

460.

[37] W.H. SETTE, A New Reverberation Time Formula, J. Acoust. Soc. Amer., 4, 193-210

(1933).

[38] G.A. SOULOUDRE, J.S. BRADLEY, Subjective Evaluation of New Room Acoustic

Measures, J. Acoust. Soc. Amer., 98, 1 (1995).

[39] K. SRODECKI, Evaluation of the Reverberation Decay Quality in Rooms Using the

Autocorrelation Function and the Cepstrum Analysis, Acustica, 80, 3, 216-225 (1994).

[40] W. STRASZEWICZ, Some Remarks on the Reverberation Time Criterion and its

Connection with Acoustical Properties of Room, Archives of Acoustics, 2, 3, 99-107 (1968).

[41] M. TOHYAMA, A. SUZUKI, Reverberation Time in an Almost-Two-Dimensional

Diffuse Field, J. Sound Vib., 111, 3, 391 -398 (1986).

[42] M. VORLÄNDER, Objective Characterization of Sound Fields in Small Rooms, 15th

Audio Eng. Soc. Int. Conf., Copenhagen, Denmark (1998).

[43] S. WEYNA, An Image of the Energetic Acoustic Field in Reduced Parallelepiped Room

Models, Acustica - Acta Acustica, 82, 1, 72-81 (1996).

[44] R.W. YOUNG, Sabine Reverberation Equation and Sound Power Calculations, J.

Acoust. Soc. Amer., 31, 7, 912-921 (1959).

FIGURE CAPTIONS

Fig. 1. Calculated reverberation time for the case where all surfaces are of low absorption.

The line shows values predicted using the New Formula, plotted against itself

Fig. 2. Calculated reverberation time for high absorption on the floor and low absorption of

remaining surfaces. The line shows values predicted using the New Formula, plotted against

itself

Fig. 3. Calculated reverberation time for evenly distributed sound absorption.

All surfaces have an absorption coefficient of 0.4

Fig. 4. Calculated reverberation times for low absorption of all room surfaces,

the overall absorption is equal to 0.02 (a so-called “live” room condition)

Fig. 5. Calculated reverberation times for high absorption of all room surfaces,

the overall absorption is equal to 0.95 (a so-called “dead” room condition)

Fig. 6. Calculated reverberation time using various calculation formulae for high

absorption of the floor and low absorption of other surfaces (there is an average absorption

applied to the ceiling)

Fig. 7. Differences between "approximated" and measured reverberation time

Fig. 8. Comparison of measured and predicted reverberation time values for a room with a

volume range from 50 to 200 m³

Fig. 9. Comparison of measured and predicted reverberation time values for a room with a

volume range from 200 to 1000 m³

Calculated Reverberation Time [s]αf = 0.02 αc = 0.02 αww = 0.02

56789

101112131415

5 6 7 8 9 10 11 12 13 14 15Reverberation Time [s]

New

RT

Form

ula

[s]

Eyring Sabine

(f = 500 Hz)

Fig. 1. Calculated reverberation time for the case where all surfaces have low absorption. The

line shows values predicted using the New Formula, plotted against itself

Calculated Reverberation Time [s]αf = 0.65 αc = 0.20 αww = 0.02

0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.5 1.0 1.5 2.0 2.5Reverberation Time [s]

New

RT

Form

ula

[s]

Eyring Sabine

(f = 500 Hz)

Fig. 2. Calculated reverberation time for high absorption on the floor and low absorption of

remaining surfaces. The line shows values predicted using the New Formula, plotted against

itself

Calculated Reverberation Time [s]αcf = 0.40 αww = 0.40

0.0

0.5

1.0

1.5

2.0

0.0 0.5 1.0 1.5 2.0

Eyring RT Formula [s]

New

RT

For

mul

a [s

]

Fig. 3. Calculated reverberation time for evenly distributed sound absorption.

All surfaces have an absorption coefficient of 0.4

0

5

10

15

20

25

30

35

40

45

0 1000 2000 3000 4000 5000 6000 7000 8000Room volume in m³

Rev

erbe

ratio

n Ti

me

in s

Sabine EyringFitzroy Millington-SetteTohyama ArauNew Formula Annex D of prEN 12354-6Eyring-Kuttruff

αww = 0.02 α cf = 0.02

Tohyama

prEN 12354-6

Fig. 4. Calculated reverberation times for low absorption of all room surfaces,

the overall absorption is equal to 0.02 (a so-called “live” room condition)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 1000 2000 3000 4000 5000 6000 7000 8000

Room volume in

Rev

erbe

ratio

n Ti

me

Sabine EyringFitzroy Millington-SetteTohyama ArauNew Formula Annex D of prEN 12354-6Eyring-Kuttruff

Sabine

αww = 0.95 αcf = 0.95

Tohyama

prEN 12354-6

low frequency condition

Fig. 5. Calculated reverberation times for high absorption of all room surfaces,

the overall absorption is equal to 0.95 (a so-called “dead” room condition)

0

2

4

6

8

10

12

14

0 1000 2000 3000 4000 5000 6000 7000 8000Room volume in m³

Rev

erbe

ratio

n Ti

me

in s

Sabine EyringFitzroy Millington-SetteTohyama ArauNew Formula Annex D of prEN 12354-6Eyring-Kuttruff

Fitzroy

αww = 0.02 α cf = 0.43

Arau

prEN 12354-6

low frequency condition

Fig. 6. Calculated reverberation time using various calculation formulae for high absorption

of the floor and low absorption of other surfaces (there is an average absorption applied to the

ceiling)

-0,5

0

0,5

1

1,5

2

2,5

3

3,5

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Room volume in m3

Rev

erbe

ratio

n Ti

me

[s]

Diff

eren

ce b

etw

een

mea

sure

d R

T an

d S

abin

e's

appr

oxim

. T60

[s

] Sabine's approxim. [s]

Measured Value (RT) [s]

Difference: Measured RT minusSabine's approxim. [s]

Fig. 7. Differences between "approximated" and measured reverberation time

Comparison of Measured and Calculated RT

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 50 100 150 200

Room volume in m³

Rev

erbe

ratio

n Ti

me

in s

Measured RT Eyring New FormulaEyring-Kuttruff Fitzroy Millington-SetteTohyama Arau Annex D of prEN 12354-6

Fig. 8. Comparison of measured and predicted reverberation time values for a room with a

volume range of from 50 to 200 m³

Comparison of Measured and Calculated RT

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 100 200 300 400 500 600 700 800 900 1000Room volume in m³

Rev

erbe

ratio

n Ti

me

in s

Measured RT Eyring New FormulaEyring-Kuttruff Fitzroy Millington-SetteTohyama Arau Annex D of prEN 12354-6

Fig. 9. Comparison of measured and predicted reverberation time values for a room

with a volume range of from 200 to 1000 m³